Abstract ObjectiveTo investigate the impact of behavioural-insight informed invitation letters on childhood influenza vaccine uptake and consent form return. Methods Schools cRCT: 2x2 factorial design comparing i) intervention letter to control (letter used in previous year) and ii) request for a reminder message to control (no request). GPs cRCT: comparison of centrally-sent intervention invitation letter with no centralised letter. Letters were sent using the Child Health Information System (CHIS) to 2 and 3 years olds. Results Schools: 1,358 schools were randomised. Data were analysed on 1,208 schools with Years 1-3 (from four providers covering 11 local authorities). Crude uptake in those receiving the intervention letter with reminder was 62.3% and without reminder 60.8%; control letter with reminder was 61.7% and without reminder 60.5%. After controlling, for example for age, the adjusted odds ratio (aOR) of vaccine uptake for the intervention letter was 1.03 (95%CI 0.97-1.08) and 1.06 (1.00-1.15) for the reminder request. For return of consent forms, the intervention letter aOR was 1.08 (1.00-1.17) and reminder request 1.09 (1.00-1.17). GPs: 257 practices from 7 CCGs were randomised. Data were available on 250 practices. CHIS recorded uptake in practices with data systems which automatically update CHIS was 44.2% in the intervention arm and 28.9% in control. In non-linked practices, crude uptake was 21.7% and 12.0% respectively. Conclusion The schools trial showed that modifying the communication had a small but not statistically significant impact on uptake. The GP trial provides further evidence of the benefit on immunisation uptake of sending invitation letters.